RFC 5124

Network Working Group J. Ott
Request for Comments: 5124 Helsinki University of Technology
Category: Standards Track E. Carrara
KTH
February 2008 Extended Secure RTP Profile for
Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/SAVPF)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
An RTP profile (SAVP) for secure real-time communications and another
profile (AVPF) to provide timely feedback from the receivers to a
sender are defined in RFC 3711 and RFC 4585, respectively. This memo
specifies the combination of both profiles to enable secure RTP
communications with feedback.

1. Introduction
The Real-time Transport Protocol, the associated RTP Control Protocol
(RTP/RTCP, RFC 3550) [1], and the profile for audiovisual
communications with minimal control (RFC 3551) [2] define mechanisms
for transmitting time-based media across an IP network. RTP provides
means to preserve timing and detect packet losses, among other
things, and RTP payload formats provide for proper framing of
(continuous) media in a packet-based environment. RTCP enables
receivers to provide feedback on reception quality and allows all
members of an RTP session to learn about each other.
The RTP specification provides only rudimentary support for
encrypting RTP and RTCP packets. Secure RTP (RFC 3711) [4] defines
an RTP profile ("SAVP") for secure RTP media sessions, defining
methods for proper RTP and RTCP packet encryption, integrity, and
replay protection. The initial negotiation of SRTP and its security
parameters needs to be done out-of-band, e.g., using the Session
Description Protocol (SDP, RFC 4566) [6] together with extensions for
conveying keying material (RFC 4567 [7], RFC 4568 [8]).
The RTP specification also provides limited support for timely
feedback from receivers to senders, typically by means of reception
statistics reporting in somewhat regular intervals depending on the
group size, the average RTCP packet size, and the available RTCP
bandwidth. The extended RTP profile for RTCP-based feedback ("AVPF")
(RFC 4585, [3]) allows session participants statistically to provide
immediate feedback while maintaining the average RTCP data rate for
all senders. As for SAVP, the use of AVPF and its parameters needs
to be negotiated out-of-band by means of SDP (RFC 4566, [6]) and the
extensions defined in RFC 4585 [3].
Both SRTP and AVPF are RTP profiles and need to be negotiated. This
implies that either one or the other may be used, but both profiles
cannot be negotiated for the same RTP session (using one SDP session
level description). However, using secure communications and timely
feedback together is desirable. Therefore, this document specifies a
new RTP profile ("SAVPF") that combines the features of SAVP and
AVPF.
As SAVP and AVPF are largely orthogonal, the combination of both is
mostly straightforward. No sophisticated algorithms need to be
specified in this document. Instead, reference is made to both
existing profiles and only the implications of their combination and
possible deviations from rules of the existing profiles are described
as is the negotiation process.

1.1. Definitions
The definitions of RFC 3550 [1], RFC 3551 [2], RFC 4585 [3], and RFC
3711 [4] apply.
The following definitions are specifically used in this document:
RTP session:
An association among a set of participants communicating with
RTP as defined in RFC 3550 [1].
(SDP) media description:
This term refers to the specification given in a single m= line
in an SDP message. An SDP media description may define only one
RTP session.
Media session:
A media session refers to a collection of SDP media descriptions
that are semantically grouped to represent alternatives of the
same communications means. Out of such a group, one will be
negotiated or chosen for a communication relationship and the
corresponding RTP session will be instantiated. If no common
session parameters suitable for the involved endpoints can be
found, the media session will be rejected. In the simplest
case, a media session is equivalent to an SDP media description
and equivalent to an RTP session.
1.2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5].
2. SAVPF Rules
SAVP is defined as an intermediate layer between RTP (following the
regular RTP profile AVP) and the transport layer (usually UDP). This
yields a two-layer hierarchy within the Real-time Transport Protocol.
In SAVPF, the upper (AVP) layer is replaced by the extended RTP
profile for feedback (AVPF).
AVPF modifies timing rules for transmitting RTCP packets and adds
extra RTCP packet formats specific to feedback. These functions are
independent of whether or not RTCP packets are subsequently encrypted
and/or integrity protected. The functioning of the AVPF layer
remains unchanged in SAVPF.

The AVPF profile derives from RFC 3550 [1] the (optional) use of the
encryption prefix for RTCP. The encryption prefix MUST NOT be used
within the SAVPF profile (it is not used in SAVP, as it is only
applicable to the encryption method specified in [1]).
The SAVP part uses extra fields added to the end of RTP and RTCP
packets and executes cryptographic transforms on (some of) the
RTP/RTCP packet contents. This behavior remains unchanged in SAVPF.
The average RTCP packet size calculation done by the AVPF layer for
timing purposes MUST take into account the fields added by the SAVP
layer.
The SRTP part becomes active only when the RTP or RTCP was scheduled
by the "higher" AVPF layer or received from the transport protocol,
irrespective of its timing and contents.
2.1. Packet Formats
AVPF defines extra packet formats to provide feedback information.
Those extra packet formats defined in RFC 4585 [3] (and further ones
defined elsewhere for use with AVPF) MAY be used with SAVPF.
SAVP defines a modified packet format for SRTP and SRTCP packets that
essentially consists of the RTP/RTCP packet formats plus some
trailing protocol fields for security purposes. For SAVPF, all RTCP
packets MUST be encapsulated as defined in Section 3.4 of RFC 3711
[4].
2.2. Extensions
Extensions to AVPF RTCP feedback packets defined elsewhere MAY be
used with the SAVPF profile provided that those extensions are in
conformance with the extension rules of RFC 4585 [3].
Additional extensions (e.g., transforms) defined for SAVP following
the rules of Section 6 of RFC 3711 [4] MAY also be used with the
SAVPF profile. The overhead per RTCP packet depends on the
extensions and transforms chosen. New extensions and transforms
added in the future MAY introduce yet unknown further per-packet
overhead.
Finally, further extensions specifically to SAVPF MAY be defined
elsewhere.

2.3. Implications from Combining AVPF and SAVP
The AVPF profile aims at -- statistically -- allowing receivers to
provide timely feedback to senders. The frequency at which receivers
are, on average, allowed to send feedback information depends on the
RTCP bandwidth, the group size, and the average size of an RTCP
packet. SRTCP (see Section 3.4 of RFC 3711 [4]) adds extra fields
(some of which are of configurable length) at the end of each RTCP
packet that are probably at least some 10 to 20 bytes in size (14
bytes as default). Note that extensions and transforms defined in
the future, as well as the configuration of each field length, MAY
add greater overhead. By using SRTP, the average size of an RTCP
packet will increase and thus reduce the frequency at which (timely)
feedback can be provided. Application designers need to be aware of
this, and take precautions so that the RTCP bandwidth shares are
maintained. This MUST be done by adjusting the RTCP variable
"avg_rtcp_size" to reflect the size of the SRTCP packets.
3. SDP Definitions
3.1. Profile Definition
The AV profiles defined in RFC 3551 [2], RFC 4585 [3], and RFC 3711
[4] are referred to as "AVP", "AVPF", and "SAVP", respectively, in
the context of, e.g., the Session Description Protocol (SDP) [3].
The combined profile specified in this document is referred to as
"SAVPF".
3.2. Attribute Definitions
SDP attributes for negotiating SAVP sessions are defined in RFC 4567
[7] and RFC 4568 [8]. Those attributes MAY also be used with SAVPF.
The rules defined in [7] and [8] apply.
SDP attributes for negotiating AVPF sessions are defined in RFC 4585
[3]. Those attributes MAY also be used with SAVPF. The rules
defined in [3] apply.
3.3. Profile Negotiation
Session descriptions for RTP sessions may be conveyed using protocols
dedicated for multimedia communications such as the SDP offer/answer
model (RFC 3264, [9]) used with the Session Initiation Protocol (SIP)
[15], the Real Time Streaming Protocol (RTSP) [10], or the Session
Announcement Protocol (SAP) [11], but may also be distributed using
email, NetNews, web pages, etc.

The offer/answer model allows the resulting session parameters to be
negotiated using the SDP attributes defined in RFC 4567 [7] and RFC
4568 [8]. In the following subsection, the negotiation process is
described in terms of the offer/answer model.
Afterwards, the cases that do not use the offer/answer model are
addressed: RTSP-specific negotiation support is provided by RFC 4567
[7] as discussed in Section 3.3.2, and support for SAP announcements
(with no negotiation at all) is addressed in Section 3.3.3.
3.3.1. Offer/Answer-Based Negotiation of Session Descriptions
Negotiations (e.g., of RTP profiles, codecs, transport addresses,
etc.) are carried out on a per-media session basis (e.g., per m= line
in SDP). If negotiating one media session fails, others MAY still
succeed.
Different RTP profiles MAY be used in different media sessions. For
negotiation of a media description, the four profiles AVP, AVPF,
SAVP, and SAVPF are mutually exclusive. Note, however, that SAVP and
SAVPF entities MAY be mixed in a single RTP session (see Section 4).
Also, the offer/answer mechanism MAY be used to offer alternatives
for the same media session and allow the answerer to choose one of
the profiles.
Provided that a mechanism for offering alternative security profiles
becomes available (as is presently under development [14]), an
offerer that is capable of supporting more than one of these profiles
for a certain media session SHOULD always offer all alternatives
acceptable in a certain situation. The alternatives SHOULD be listed
in order of preference and the offerer SHOULD prefer secure profiles
over non-secure ones. The offer SHOULD NOT include both a secure
alternative (SAVP and SAVPF) and an insecure alternative (e.g., AVP
and AVPF) in the same offer as this may enable bidding down and other
attacks. Therefore, if both secure and non-secure RTP profiles are
offered (e.g., for best-effort SRTP [14]), the negotiation signaling
MUST be protected appropriately to avoid such attacks.
If an offer contains multiple alternative profiles, the answerer
SHOULD prefer a secure profile (if supported) over non-secure ones.
Among the secure or insecure profiles, the answerer SHOULD select the
first acceptable alternative to respect the preference of the
offerer.
If a media description in an offer uses SAVPF and the answerer does
not support SAVPF, the media session MUST be rejected.

If a media description in an offer does not use SAVPF but the
answerer wants to use SAVPF, the answerer MUST reject the media
session. The answerer MAY provide a counter-offer with a media
description indicating SAVPF in a subsequently initiated offer/answer
exchange.
3.3.2. RTSP-Based Negotiation of Session Descriptions
RTSP [10] does not support the offer/answer model. However, RTSP
supports exchanging media session parameters (including profile and
address information) by means of the Transport header. SDP-based key
management as defined in RFC 4567 [7] adds an RTSP header (KeyMgmt)
to support conveying a key management protocol (including keying
material).
The RTSP Transport header MAY be used to determine the profile for
the media session. Conceptually, the rules defined in Section 3.3.1
apply accordingly. Detailed operation is as follows: An SDP
description (e.g., retrieved from the RTSP server by means of
DESCRIBE) contains the description of the media streams of the
particular RTSP resource.
The RTSP client MUST select exactly one of the profiles per media
stream it wants to receive. It MUST do so in the SETUP request. The
RTSP client MUST indicate the chosen RTP profile by indicating the
profile and the full server transport address (IP address and port
number) in the Transport header included in the SETUP request. The
RTSP server's response to the client's SETUP message MUST confirm
this profile selection or refuse the SETUP request (the latter of
which it should not do after offering the profiles in the first
place).
Note: To change any of the profiles in use, the client needs to
tear down this media stream (and possibly the whole RTSP
session) using the TEARDOWN method and re-establish it using
SETUP. This may change as soon as media updating (similar to a
SIP UPDATE or re-INVITE) becomes specified.
When using the SDP key management [7], the KeyMgmt header MUST be
included in the appropriate RTSP messages if a secure profile is
chosen. If different secure profiles are offered in the SDP
description (e.g., SAVP and SAVPF) and different keying material is
provided for these, after choosing one profile in the SETUP message,
only the KeyMgmt header for the chosen one MUST be provided. The
rules for matching KeyMgmt headers to media streams according to RFC
4567 [7] apply.

3.3.3. Announcing Session Descriptions
Protocols that do not allow negotiating session descriptions
interactively (e.g., SAP [11], descriptions posted on a web page or
sent by mail) pose the responsibility for adequate access to the
media sessions on the initiator of a session.
The initiator SHOULD provide alternative session descriptions for
multiple RTP profiles as far as acceptable to the application and the
purpose of the session. If security is desired, SAVP may be offered
as alternative to SAVPF -- but AVP or AVPF sessions SHOULD NOT be
announced unless other security means not relying on SRTP are
employed.
The SDP attributes defined in RFC 4567 [7] and RFC 4568 [8] may also
be used for the security parameter distribution of announced session
descriptions.
The security scheme description defined in RFC 4568 [8] requires a
secure communications channel to prevent third parties from
eavesdropping on the keying parameters and manipulation. Therefore,
SAP security (as defined in RFC 2974 [11]), S/MIME [12], HTTPS [13],
or other suitable mechanisms SHOULD be used for distributing or
accessing these session descriptions.
3.3.4. Describing Alternative Session Profiles
SAVP and SAVPF entities MAY be mixed in the same RTP session (see
also Section 4) and so MAY AVP and AVPF entities. Other combinations
-- i.e., between secure and insecure profiles -- in the same RTP
session are incompatible and MUST NOT be used together.
If negotiation between the involved peers is possible (as with the
offer/answer model per Section 3.3.1 or RTSP per Section 3.3.2),
alternative (secure and non-secure) profiles MAY be specified by one
entity (e.g., the offerer) and a choice of one profile MUST be made
by the other. If no such negotiation is possible (e.g., with SAP as
per Section 3.3.3), incompatible profiles MUST NOT be specified as
alternatives.
The negotiation of alternative profiles is for further study.
RTP profiles MAY be mixed arbitrarily across different RTP sessions.

3.4. Examples
This section includes examples for the use of SDP to negotiate the
use of secure and non-secure profiles. Depending on what keying
mechanism is being used and how it parameterized, the SDP messages
typically require integrity protection and, for some mechanisms, will
also need confidentiality protection. For example, one could say
integrity protection is required for the a=fingerprint of Datagram
Transport Layer Security - Secure Real-time Transport Protocol
(DTLS-SRTP) [16], and confidentiality is required for RFC 4568 [8]
(Security Descriptions) a=crypto.
Example 1: The following session description indicates a secure
session made up from audio and dual tone multi-frequency (DTMF) for
point-to-point communication in which the DTMF stream uses Generic
NACKs. The key management protocol indicated is MIKEY. This session
description (the offer) could be contained in a SIP INVITE or 200 OK
message to indicate that its sender is capable of and willing to
receive feedback for the DTMF stream it transmits. The corresponding
answer may be carried in a 200 OK or an ACK. The parameters for the
security protocol are negotiated as described by the SDP extensions
defined in RFC 4567 [7].
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Media with feedback
t=0 0
c=IN IP4 host.example.com
m=audio 49170 RTP/SAVPF 0 96
a=rtpmap:0 PCMU/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-16
a=rtcp-fb:96 nack
a=key-mgmt:mikey uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
Example 2: This example shows the same feedback parameters as example
1 but uses the secure descriptions syntax [8]. Note that the key
part of the a=crypto attribute is not protected against eavesdropping
and thus the session description needs to be exchanged over a secure
communication channel.
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Media with feedback
t=0 0
c=IN IP4 host.example.com
m=audio 49170 RTP/SAVPF 0 96
a=rtpmap:0 PCMU/8000

Transport: RTP/SAVPF;unicast;dest_addr=":53012"/":53013";
src_addr="192.0.2.15:60000"/"192.0.2.15:60001"
KeyMgmt: prot=mikey;url="rtsp://movies.example.org/example";
data="ushdgfdhgfuiweyfhjsgdkj2837do7eWsnDSJD..."
Accept-Ranges: NPT, SMPTE
Example 5: The following session description indicates a multicast
audio/video session (using PCMU for audio and either H.261 or H.263+)
with the video source accepting Generic NACKs for both codecs and
Reference Picture Selection for H.263. The parameters for the
security protocol are negotiated as described by the SDP extensions
defined in RFC 4567 [7], used at the session level. Such a
description may have been conveyed using the Session Announcement
Protocol (SAP).
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Multicast video with feedback
t=3203130148 3203137348
a=key-mgmt:mikey uiSDF9sdhs7494ghsd/dhsoKkdOokdo7eWsnDSJD...
m=audio 49170 RTP/SAVP 0
c=IN IP4 224.2.1.183
a=rtpmap:0 PCMU/8000
m=video 51372 RTP/SAVPF 98 99
c=IN IP4 224.2.1.184
a=rtpmap:98 H263-1998/90000
a=rtpmap:99 H261/90000
a=rtcp-fb:* nack
a=rtcp-fb:98 nack rpsi
4. Interworking of AVP, SAVP, AVPF, and SAVPF Entities
The SAVPF profile defined in this document is a combination of the
SAVP profile [4] and the AVPF profile [3] (which in turn is an
extension of the RTP profile as defined in RFC 3551 [2]).
SAVP and SAVPF use SRTP [4] to achieve security. AVP and AVPF use
plain RTP [1] and hence do not provide security (unless external
security mechanisms are applied as discussed in Section 9.1 of RFC
3550 [1]). SRTP and RTP are not meant to interoperate; the
respective protocol entities are not supposed to be part of the same
RTP session. Hence, AVP and AVPF on one side and SAVP and SAVPF on
the other MUST NOT be mixed.
RTP entities using the SAVP and the SAVPF profiles MAY be mixed in a
single RTP session. The interworking considerations defined in
Section 5 of RFC 4585 [3] apply.

5. Security Considerations
The SAVPF profile inherits its security properties from the SAVP
profile; therefore, it is subject to the security considerations
discussed in RFC 3711 [4]. When compared to SAVP, the SAVPF profile
does not add or take away any security services.
There is a desire to support security for media streams and, at the
same time, for backward compatibility with non-SAVP(F) nodes.
Application designers should be aware that security SHOULD NOT be
traded for interoperability. If information is to be distributed to
closed groups (i.e., confidentially protected), it is RECOMMENDED not
to offer alternatives for a media session other than SAVP and SAVPF
as described in Sections 3.3 and 3.4, unless other security
mechanisms will be used, e.g., the ones described in Section 9.1 of
RFC 3550 [1]. Similarly, if integrity protection is considered
important, it is RECOMMENDED not to offer the alternatives other than
SAVP and SAVPF, unless other mechanisms are known to be in place that
can guarantee it, e.g., lower-layer mechanisms as described in
Section 9 of RFC 3550 [1].
Offering secure and insecure profiles simultaneously may open to
bidding down attacks. Therefore, such a mix of profile offer SHOULD
NOT be made.
Note that the rules for sharing master keys apply as described in RFC
3711 [4] (e.g., Section 9.1). In particular, the same rules for
avoiding the two-time pad (keystream reuse) apply: a master key MUST
NOT be shared among different RTP sessions unless the SSRCs used are
unique across all the RTP streams of the RTP sessions that share the
same master key.
When 2^48 SRTP packets or 2^31 SRTCP packets have been secured with
the same key (whichever occurs before), the key management MUST be
called to provide new master key(s) (previously stored and used keys
MUST NOT be used again), or the session MUST be terminated.
Different media sessions may use a mix of different profiles,
particularly including a secure profile and an insecure profile.
However, mixing secure and insecure media sessions may reveal
information to third parties and thus the decision to do so MUST be
in line with a local security policy. For example, the local policy
MUST specify whether it is acceptable to have, e.g., the audio stream
not secured and the related video secured.

The security considerations in RFC 4585 [3] are valid, too. Note in
particular, applying the SAVPF profile implies mandatory integrity
protection on RTCP. While this solves the problem of false packets
from members not belonging to the group, it does not solve the issues
related to a malicious member acting improperly.
6. IANA Considerations
The following contact information shall be used for all registrations
included here:
Contact: Joerg Ott
mail: jo@acm.org
tel: +358-9-451-2460
The secure RTP feedback profile, as a combination of Secure RTP and
the feedback profile, has been registered for the Session Description
Protocol (specifically the type "proto"): "RTP/SAVPF".
SDP Protocol ("proto"):
Name: RTP/SAVPF
Long form: Secure RTP Profile with RTCP-based Feedback
Type of name: proto
Type of attribute: Media level only
Purpose: RFC 5124
Reference: RFC 5124
All the SDP attributes defined for RTP/SAVP and RTP/AVPF are valid
for RTP/SAVPF, too.
7. Acknowledgements
This document is a product of the Audio-Visual Transport (AVT)
Working Group of the IETF. The authors would like to thank Magnus
Westerlund, Colin Perkins, and Cullen Jennings for their comments.
8. References
8.1. Normative References
[1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[2] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control", STD 65, RFC 3551, July 2003.

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